cpan_home vs. build_root & locking

I'm not sure I understand the reason for the lock file
placed in cpan_home.  If I have two different architectures
with different build directories, I'm not sure I understand
where the conflict is -- is it in storing/listing Bundles,
the history file, the Metafile or the shared source?

I can understand private history, but the Bundles aren't
updated often, the sources should be fairly static (unless
new files are downloaded), The metafile would be updated about
once a day (default). 

If the build directories are separate, why would there be a
need for locking more in interactive use vs. batch use?

Thanks for insights...
Linda